Screwdriving and Riveting

Screwdriving is a deceptively simple process. All you have to do is insert a fastener and apply torque, right? In fact, a lot can go wrong, and there are many variables to consider. Is the driver speed right? What about the torque and angle settings? Is the screw itself the best fastener for the job?

Knowing how to troubleshoot screwdriving applications can improve cycle times and ensure product quality, while saving engineers countless hours of frustration.

Numerous Variables

Screwdriving is a complex process with numerous variables that must be considered to produce a high quality joint.

“It is a sensitive process that is time consuming and critical,” says Boris Baeumler, vice president of technology at Deprag Inc. “Some [problematic trends in screwdriving] include joints having higher variation due to composite materials; more self-tapping fasteners; less product testing prior to production; and rapid design methods.

“As design engineers use more self-tapping fasteners, even in critical applications, it’s become more important to ensure that these fasteners still produce the required clamp load on the joint, rather than using too much torque to produce the threads,” warns Baeumler. “To verify this and to collect data, we offer the unique process of CFC (clamp force control). This ensures that the correct amount of torque is used for clamping the product.

“Another challenge is tighter screw locations,” explains Baeumler. “Due to continuously shrinking parts, access to screw locations is getting worse. To overcome this, we use a lot of drive systems that present the fastener to the driving locations with vacuum to allow access to zero clearance areas.

“Because there are so many variables, it’s usually a good idea to use automation for screwdriving whenever possible,” Baeumler points out. “By using automation, these variables are eliminated, which simplifies the process and reduces equipment cost.”

According to Baeumler and other experts, engineers should always review product designs before releasing them for manufacturing. Unfortunately, many products are created without experienced engineers carefully reviewing joint designs and optimizing them for assembly.

“The speed of rolling out new products has increased,” says Jarrod Neff, marketing manager at Visumatic Industrial Products Inc. “Today, it’s not uncommon to be working on tooling for a new product design that’s not frozen yet.”

Fastener Quality

No matter what type of screwdriving tool is used or what type of product is assembled, poor fastener quality can cause numerous throughput issues. For instance, problems with thread cutting and thread forming create variances. Small screws that are out of tolerance will cause big headaches.

Many engineers find out the hard way that cost and quality are not the same. Often, the cheapest screws are not the best solution in the long term.

“Fastener quality is one aspect of assembly operations that’s often overlooked,” says Kevin Buckner, director of engineering at Design Tool Inc. “When a manufacturer is using automation or handheld autofeed screwdriving equipment, quality issues with fasteners, such as size variance, eccentricity problems between the head and shank, bent screws, or partially formed screws, can cause feeding problems and downtime.”

Poor product designs also cause many screwdriving slowdowns. “The biggest mistake that engineers make when attempting to improve throughput is not considering the screwdriving process when designing the components,” explains Buckner. “Screws will often be placed in locations that prevent either handheld or automated equipment from being used, which makes improving throughput very difficult.”

Sometimes, all it takes is a minor alteration. If engineers are willing to make changes to the material, the fastener or the application, they can improve assembly speed.

“Adding features to a part, such as a side pocket or a recess that can be used to align tooling, can make a big difference in throughput,” says Baeumler. “A design may feature a screw that has a square drive on it that doesn’t engage well with the bit. Changing [the head] to a Torx or a cross-recess design can help.”

According to Baeumler, fastener size also affects throughput. “Generally, the larger the fastener, the slower the cycle time,” he points out. “It’s usually easier to improve throughput with smaller screws.

“However, small fasteners are always a challenge,” warns Baeumler. “To help with this, we offer mini screw feeders that pre-orient the screw to allow for easy pickup. We also offer blow feeders for very small screws.

“Quality control with dedicated sequences can also be challenging,” says Baeumler. “If multiple screws need to be driven in a dedicated sequence to ensure proper part assembly, operators require guidance to ensure this is accomplished.”

Speed and Torque

The type of tool being used can influence throughput. For instance, operators can typically drive a screw faster with pneumatic drivers vs. electric drivers. That’s because pneumatic tools tend to have higher spindle speeds.

Throughput is synonymous with cycle time—the time required to produce a part or complete a process. It is often governed by the slowest machine in a workcell. However, engineers face many challenges when attempting to boost throughput in screwdriving applications.

“The challenges are reducing the time required to drive the screw, improving the joint quality (eliminating stripped or partially driven screws), and reducing waste from lost or dropped fasteners,” says Buckner.

All things being equal, the easiest way to improve throughput is to increase the spindle speed on screwdrivers. For instance, if torque requirements can be met, the speed of the driver can be changed from 1,000 rpm to 2,000 rpm.

However, spindle speed is not always a one-stop solution. One of the biggest mistakes that engineers make when attempting to improve throughput in screwdriving applications is increasing the speed of the driver. Increased speed can impact the clamp load in the joint, which can cause failures.”

“Just turning up the dial can be a big mistake,” warns Baeumler. “Some applications just don’t allow for that, such as plastic screws.

“Some people will go from 700 rpm to 1400 rpm, because they think it will double their cycle time,” warns Baeumler. “But, in reality, that ‘solution’ may create problems. Because there are so many other variables involved with screwdriving, increasing spindle speed is just a small factor.”

Screws have a certain preferred forward advance velocity and bit pressure that will ensure that the torque readings being taken are true to what’s actually occurring on the screw head. By over-speeding the bit, it’s possible to damage the screw threads or the parts being assembled.

By applying too much downward force, the screw thread contact and prevailing torque can be artificially raised, causing false torque readings which can manifest into unseated screws. By using the correct bit-speed-to-thread-pitch ratio, and just enough pressure to keep the driver bit engaged, operators can better trust any torque reading taken and greatly reduce the risk of damaging the screw thread in the process.

“Operators are not always taught about torque, speed, and different types of joints and how that can affect the actual result of [the screwdriving process],” says Rick Wagner, U.S. East Coast and Canada sales manager at ASG, Div. of Jergens Inc., “[I once] worked at a manufacturer where I was told to use a specific air tool that was preset and calibrated to 20 inch-pounds, running 2000 rpm, on a hard joint. On many occasions I would break bolts without a real understanding why, and we scrapped a good amount of frames because of that.

“The issue I encountered was the driver was calibrated on a softer run down adapter on a torque tester at 20 inch-pounds,” recalls Wagner. “When the driver was run on the actual assembly, the joint was a lot harder and achieving final torque would happen a lot sooner with less degrees of rotation to get there.

“When you have a hard joint and the tool is trying to shut off at 20 inch-pounds running at 2000 rpm, the probability of stopping at 20 inch-pounds is highly unlikely,” warns Wagner. “It’s like trying to stop your car while driving 65mph five feet from a brick wall; it will be impossible and you will blow through the wall.

“So, the final result we were seeing was the final actual torque applied to the fastener and assembly," says Wagner. "It was well higher than the actual required torque, which led us to exceed the yield point of the fastener.”

Materials Matter

Different types of materials can provide challenges in obtaining the correct fastener torque. For example, if the fastener is initially cutting threads, but the final torque value is low, it may require a higher torque setting to cut the threads than the final tightening specification.

“If the material is soft or easily stripped, obtaining the proper torque will be very difficult,” warns Buckner. “Also, when the same part is installed on multiple material types, maintaining proper torque on all material types is difficult as well. In both of these scenarios, we would recommend our DTI 5000 automatic screwdriving system.

“When driving screws into wood, the wood type and density, knots or other imperfections in the [material] can cause similar problems with obtaining the correct torque, particularly if a pneumatic clutch type driver is used,” notes Buckner. “In this case, the DTI 5000 [would be ideal]. Its drive to depth feature allows a wood screw to be driven to flush, or slightly below the work surface, on every cycle without prematurely reaching torque or driving too deep into the material.”

According to Buckner, incorrect driver speed can cause multiple problems when using automatic screwfeeding systems. “When driving screws into polymers such as plastic or nylon, if the speed is too high, melting around the fastener can occur due to excessive heat, reducing fastener clamp load,” he points out.

“With self-drilling screws, drivers that are too slow will require higher force to get the screw drill point to cut into the material, and drivers that are too fast can damage the screw drill point, preventing the screw from cutting into the material,” says Buckner. “In addition, driver speed that is too high can lead to problems with over-tightening or stripping screws, as the driver clutch or controller may not be able to overcome the driver rotational inertia in time to regulate the torque.

“Use of incorrect parameters in DC driver controller programs will lead to problems with achieving fastener torque that is within the specifications,” adds Buckner. “Many times, run-down speed and torque must be reduced to allow the DC driver to enter the final tightening phase without exceeding torque or angle limits. If these parameters are not set correctly, the fastening process will fail to obtain a good joint.”

Operator Errors

Many screwdriving problems are caused by simple operator error. And, as some veteran engineers have discovered, most of the same mistakes from the past are still being made today. That’s often due to a lack of training caused by labor shortages and high turnover rates.

“For automatic screwdriving systems, one of the biggest mistakes that operators make is not maintaining adequate pressure on the driver to keep the bit engaged with the screw while the screw is being driven, resulting in the bit camming out or surface driving the screw,” says Buckner. “Either condition may cause improper tightening and premature bit wear, along with damage to screw heads.

“Engineers should consider the torque limitations of the screw drive type, as well as selecting a screw design that allows for full bit engagement,” suggests Buckner. “Allowing adequate room around the screw location to make it easier for operators to access is critical as well. Screw locations that are difficult to access will inevitably have more problems driving screws.”

“Some operators, especially if they’re new, don’t always allow the screw run down process to complete,” says Visumatic’s Neff. “They drive a screw maybe 95 percent of the way and then lift up before the cycle is complete. That will leave them with either a high screw or a joint that’s not fully tight.”

“The biggest mistake that operators make is not allowing the screwdriver to finish its job and drive to torque,” adds Deprag’s Baeumler. “To prevent this, we offer a process control function in all of our platforms. This ensures that an operator lets the screwdriver finish one process before they can start the next.”

Another mistake, especially with standard mechanical clutch electric or pneumatic drivers, is grabbing the wrong tool for the application. Some operators also make changes to the driver to increase torque output, thinking it will make things easier.

Inspection System

“I’ve seen many facilities that have five or six screwdrivers hanging above a workbench all calibrated to a different torque spec,” says Wagner. “I have watched operators just reach for one and then get called out for running the wrong driver for a specific torque.

“I have also watched operators encounter running or prevailing torque due to thread issues or debris, then turn the torque up on the driver just to get the fastener to drive all the way in, and then occasionally break screws or parts in an assembly,” notes Wagner.

“One of the biggest changes that engineers can make is switching from standard mechanical clutch electric or pneumatic drivers to systems like our X-PAQ series drivers,” claims Wagner. “These DC-electric controlled tools feature a built in transducer. The system can be programmed with 99 different tasks and can be locked out so that operators are unable to make any changes to the programs.

“The system can use a bar code scanner, inputs or bolt sequence to make the operator utilize the proper torque for each fastener,” explains Wagner. “There will be no need to have multiple drivers hanging over a workstation. You can have one system control all your assembly needs.

“Our X-PAQ SD2500 series drivers and systems are [a good way] to address fastening issues,” Wagner points out. “With the internal graph of a rundown, you can use it to [enable] the engineering team to see the actual yield of the joint to help figure out the required torque needed [for an application].

“You can automatically calculate the prevailing or running torque through the fastening run down to accurately ensure that you will seat your fastener to the required final torque,” says Wagner. “Multistep run down options can help overcome the possibility of cross threading, thread engagement and speed downshift to increase the accuracy of the final torque phase.”